Improving the pawn structure on the kingside. Black can play for hours on end before finally demonstrating a plan, which makes defense unpleasant. As expected, the game went on for many moves without a change in the assessment.
30. d4b4  30... b8b4  31. a3a8  31... f8a8  32. f3a8 Without rooks or bishops, the endgame is easily drawn, and Gukesh wouldn't even try too much. But with rooks and bishops, it's possible to play with some hope of winning, although it would take a lot of optimism to really believe that the last game of a world championship match would end in a decisive result in such a position.